[Rutherford]

1.A and B are elements and in room temperature gasses. They react with a explosion produced and on -250°C they make substance B that produces in water a weak acidic solution. The boiling point of B is 19.5°C and the dense of it's vapor to hydrogen varies from 10 to 60 (depends on the temperature, it is lower on higher temperatures). Write the formula of substance B.

2.A colorless gass A, whose density is 3,4% less that the density of air, burned out in oxygen. The products are passed through a 10% solution of NaHCO3. The mass of water didn't change and substance B is made. Write the formula of A and B.

I have dozens of similar problems (with guessing an unknown substance), but I couldn't solve a single one. I don't know where to start from, how to use the informations that are given in the problems and how the guess the unknown substances. Some guidelines would be most appreciated.

[Arkcon]

Yeah.  Number 1 is kind of tough.  But lets start:  A and B are elements, and gases at room temp.  So go to the periodic table and write those down.  It would truly suck if the question said A and B are elements, metals and solid at room temp.  So lets at least be grateful for something.  Try to use the other info in like fashion.

[Rutherford]

It dawned to me. I tried with the elimination system. First to separate the non-metals that are gasses at room temperature (H,inert gasses,N,O,F,Cl). A and B react so I eliminate the inert gasses. From the other informations I could only think off how to use this:"weak acidic solution" and the boiling point. Weak acidic solution produce hydrides of elements from the VIa group and the VIIa group (they are more acidic) and the acidity increaces in the group.One of the substances is hydrogen then. From the VIa group, oxygen is the only gass that produces a hydride (water) but because of the hydrogen bond the boiling temperature can't be 19.5°C (it is 100°C). Only hydrogen-halides left (HF and HCl, because only Cl and F are gasses on room temperature) and HF is a weaker acid than HCl so the answer is HF and it is the right answer. The problem is that I won't be able to use the periodic system.
And I can't solve the second one.

[AWK]

CO₂ does not react with NaHCO₃

[Rutherford]

Right, and when calculating from the density: 29-29*0,034=28g/mol A=CO (12+16).
But see this:
When dissoluting 6,714g of a metal M in 25% nitrate acid, 1500cm3 of a gass mixture is made (normal conditions), 39% of the volume is NO, 53.81% is N2O and there is made also a substance A. What is that substance A and what was the starting metal?
And thanks for the helpm, I could finish now alone a few problems.

[Borek]

Questions are so ambiguous to me I wonder if it is a problem with questions or with your translation.

The mass of water didn't change and substance B is made.

I guess you mean "mass of the solution didn't change". If so, I don't see where the substance B comes from.

When dissoluting 6,714g of a metal M in 25% nitrate acid, 1500cm3 of a gass mixture is made (normal conditions), 39% of the volume is NO, 53.81% is N2O and there is made also a substance A.

What is substance A - is it a gas that makes up remaining 100-39-55.81%, or is it a salt produced?

[Rutherford]

I did some mistakes by translating the first one,"the mass of water didn't change" this is true, and the substance B isn't made, it is in the solution after passing the products of the burning out-here is the mistake.
For the second one, A is a gass.
Sorry for the bad translation.

[Rutherford]

Just wanted to ask if someone knows some good sites or e-books with similar examples (to determine an unkwnown substance). I've run out of problems and I think that I didn't practice those enough.